linux-kernel - Re: [PATCH v7 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

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Message-ID: <Y7cToj5mWd1ZbMyQ@arm.com>
Date:   Thu, 5 Jan 2023 18:14:58 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
        will@...nel.org, anshuman.khandual@....com,
        linux-doc@...r.kernel.org, corbet@....net, peterz@...radead.org,
        arnd@...db.de, punit.agrawal@...edance.com,
        linux-kernel@...r.kernel.org, darren@...amperecomputing.com,
        yangyicong@...ilicon.com, huzhanyuan@...o.com, lipeifeng@...o.com,
        zhangshiming@...o.com, guojian@...o.com, realmz6@...il.com,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, Barry Song <21cnbao@...il.com>,
        wangkefeng.wang@...wei.com, xhao@...ux.alibaba.com,
        prime.zeng@...ilicon.com, Barry Song <v-songbaohua@...o.com>,
        Nadav Amit <namit@...are.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v7 2/2] arm64: support batched/deferred tlb shootdown
 during page reclamation

On Thu, Nov 17, 2022 at 04:26:48PM +0800, Yicong Yang wrote:
> It is tested on 4,8,128 CPU platforms and shows to be beneficial on
> large systems but may not have improvement on small systems like on
> a 4 CPU platform. So make ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH depends
> on CONFIG_EXPERT for this stage and make this disabled on systems
> with less than 8 CPUs. User can modify this threshold according to
> their own platforms by CONFIG_NR_CPUS_FOR_BATCHED_TLB.

What's the overhead of such batching on systems with 4 or fewer CPUs? If
it isn't noticeable, I'd rather have it always on than some number
chosen on whichever SoC you tested.

Another option would be to make this a sysctl tunable.

>  .../features/vm/TLB/arch-support.txt          |  2 +-
>  arch/arm64/Kconfig                            |  6 +++
>  arch/arm64/include/asm/tlbbatch.h             | 12 +++++
>  arch/arm64/include/asm/tlbflush.h             | 52 ++++++++++++++++++-
>  arch/x86/include/asm/tlbflush.h               |  5 +-
>  include/linux/mm_types_task.h                 |  4 +-
>  mm/rmap.c                                     | 10 ++--

Please keep any function prototype changes in a preparatory patch so
that the arm64 one only introduces the arch specific changes. Easier to
review.

> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> +{
> +	/*
> +	 * TLB batched flush is proved to be beneficial for systems with large
> +	 * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
> +	 * is cheap on small systems which may not need this feature. So use
> +	 * a threshold for enabling this to avoid potential side effects on
> +	 * these platforms.
> +	 */
> +	if (num_online_cpus() < CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
> +		return false;

The x86 implementation tracks the cpumask of where a task has run. We
don't have such tracking on arm64 and I don't think it matters. As
noticed/described in this series, the bottleneck is the actual DSB
synchronisation (which sends a DVM Sync message to all the other CPUs
and waits for a DVM Complete response). So I think it makes sense not to
bother with an mm_cpumask(). What this patch aims to optimise is
actually the number of DSBs issued on an SMP system by
ptep_clear_flush().

The DVM is not an architected concept (well, it's part of AMBA AXI). I'd
be curious to know how such patch behaves on Apple's M1/M2 hardware. My
preference would be to have this always on for num_online_cpus() > 1 if
there's no overhead.

-- 
Catalin